1. Technical Field of the Invention
The present invention relates to stators for starter motors mainly for use in starting up automotive engines and, more particularly, to a yoke assembly for a stator of a starter motor having a cylindrical yoke whose inner peripheral wall carries thereon magnet springs each of which supports a stator element composed of a permanent magnet, serving as a main pole, and an auxiliary pole placed adjacent to the permanent magnet on an increasing magnetic flux side thereof.
2. Description of the Related Art
Attempts have heretofore been made to use a D.C. motor as a magnetic-field type starter employing a permanent magnet. With such a motor, a stator includes a cylindrical yoke having an inner peripheral wall that carries thereon a plurality of axially extending magnet springs (resilient holder elements) each formed in a nearly U-shape in cross section and opened toward a center of the yoke. The magnet springs support thereon stator elements, respectively. With such a structure, if each of the stator elements employs an auxiliary pole held in contact with the permanent magnet (main pole) on an increasing magnetic flux side thereof, the starter motor can have an improved rotational speed in a low current area without causing any drop in output torque in a high current area. This makes it possible to provide improved startability at a normal temperature as disclosed in Japanese Patent Application Publication 9-168245.
With the starter motors of the related art including the stator element comprised of the main poles and the auxiliary poles, a yoke assembling has been performed by assembling the yoke in two methods including: a method of fixedly securing the magnet springs on the yoke at the inner peripheral wall thereof after which the main pole and the auxiliary pole are simultaneously assembled to the yoke; and another method of preliminarily fixing the auxiliary poles onto the yoke by welding for assembly. Therefore, this results in a need to take an assembling process that is different from a yoke assembling for a structure employing only main poles in the absence of the auxiliary poles.
As a result, when manufacturing starter motors of two types such as a starter motor on which the stator elements having the auxiliary poles are mounted and a starter motor on which only the main poles are mounted, plural assembling processes are needed, resulting in a difficulty of achieving a reduction in manufacturing cost on mass production.